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IF choices

Discussion in 'General Electronics Discussion' started by Pritam, Mar 24, 2016.

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  1. Pritam

    Pritam

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    Mar 24, 2016
    Hello dears,
    My question is, Why IF (Intermediate frequency) doesn't lie within RF (radio frequency) range.
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    hi
    welcome to EP :)

    they do, where did you think they were ?


    Dave
     
  3. dorke

    dorke

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    Jun 20, 2015
    You need to rephrase your question.
    Did you mean to ask,
    Why is the IF frequency outside the frequency band of a receiver?
     
  4. davenn

    davenn Moderator

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    not sure if that helps ;)

    for some multi band radios there can be multiple IF's and can be within the range of normal reception


    Dave
     
  5. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    The purpose of an IF is to allow tight control of the bandwidth and gain of the IF amplifier, thus improving the selectivity of the receiver. As @davenn mentioned, some receivers (especially higher frequency receivers) have two or more IF amplifiers because of the requirements of the local oscillator and mixer circuitry. It all depends on what kind of receiver you are designing. If the receiver is to tune from DC to light, a lot more than one IF will be necessary.
     
    davenn likes this.
  6. davenn

    davenn Moderator

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    Sep 5, 2009
    and also, to bring the frequency down to one that is more easily demodulated to recover the information
    455kHz or thereabouts is a very common base IF freq. and has been since the dawn of radio receivers

    The next very common combination is 1st IF of 10.7 MHz then down to 2nd IF of 455kHz

    One radio I have has a number of different IF freq's depending on what band is being used
    I have highlighted them in red

    IF freqs.GIF


    Dave
     
  7. ramussons

    ramussons

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    Jun 10, 2014
    Common combination? :confused: The only place where I've seen this is in a SCPC satellite receiver channel modem :D
     
  8. davenn

    davenn Moderator

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    then you haven't done much RF work ! ;)

    it VERY common in commercial comms equip
     
  9. dorke

    dorke

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    Jun 20, 2015
    An interesting link ,the history part especially IF.

    There is also the term "zero IF" ,
    which actually means a "direct receiver".

    And another note on the issue:
    One important parameter of receivers is " IF rejection ratio" .
     
  10. ramussons

    ramussons

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    Jun 10, 2014
    I linked the term "common" to common broadcast radios :D not to professional equipment. Did'nt think the OP had gone to that level of explanation.
     
  11. davenn

    davenn Moderator

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    yup a few ham friends have played with zero IF ( direct conversion ) haven't done so myself

    you might find this interesting .....
    http://lea.hamradio.si/~s53mv/zifssb/block.html

    That's a VERY limited view ;)
    and I wasn't referring to professional equip ... I said commercial comms
    think of radio gear used in taxi's, couriers and a dozen other similar uses

    In older, and occasionally slightly newer microwave linking gear, 70MHz was/is
    the common base band IF


    Dave
     
    Last edited: Mar 25, 2016
  12. hevans1944

    hevans1944 Hop - AC8NS

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    Not sure about other IFs, but 455 kHz was used in about a zillion superhet AM broadcast receivers dating back to the early part of the 20th Century. The reason it became so popular was a self-fulfilling feedback from the number of receivers using it and the availability of "standard" IF transformers for inter-stage coupling with built-in tuning capacitors and slug adjustments for the inductor core.

    One of the first test instruments that I purchased back in the 1950s or early 1960s (I forget exactly when), right after assembling an EICO kit oscilloscope, was a sweep frequency generator (with markers!) that I used to adjust the bandwidth of television video IF amplifiers. This was almost never necessary unless a customer had helpfully "tightened down" all the slugs in the IF transformers in their misguided attempts at repair... usually after running all the vacuum tubes through a drug-store tube tester and replacing all the "bad" tubes.

    Were it not for transistors, drug stores would still be raking in big bux selling replacement tubes their "testers" had discovered were "bad". Transistors also spelled the demise of TV repair shops because transistors hardly ever failed. I managed to get out of the TV repair business after graduating from high school by joining the Air Force. The Air Force also introduced me to better toys, which carried over to an engineering career after my four-year hitch was up..

    Another "favorite" IF is 10.7 MHz, used in maybe a half-zillion superhet FM broadcast receivers, also dating back to the early part of the 20th Century.

    This Wikipedia article has a pretty good overview of how, when, where, and why IFs are used in communications receivers, along with a list of IFs that have been used in commercial equipment. The designer, of course, is free to choose whatever IF will suit the purpose of their design.
     
  13. Pritam

    Pritam

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    Mar 24, 2016
    Correct. Thank u.
     
  14. dorke

    dorke

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    Jun 20, 2015

    "The Air Force also introduced me to better toys, which carried over to an engineering career after my four-year hitch was up.."

    Does the names CEI and WJ ring a bell?

    "Another "favorite" IF is 10.7 MHz, used in maybe a half-zillion superhet FM broadcast receivers, also dating back to the early part of the 20th Century."

    A small correction:
    Commercial broadcast FM radios is an after WWII thing.


    "This Wikipedia article has a pretty good overview of how, when, where, and why IFs are used in communications receivers, along with a list of IFs that have been used in commercial equipment."

    I linked to that one in #9;)
    Some common frequencies absent are 10MHz and 2MHz .
    and the "modern " digital TVs ones at 36MHZ and 44MHz .
     
  15. davenn

    davenn Moderator

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    for simple one or two band radios, this is commonly the case. But in multi band radios as the one I described earlier and another one I own
    that have general coverage receivers ... ALL the IF freq's fall within the covered range of the receiver, even the 455kHz, as the radio
    is capable of going down to 150 kHz in receive mode.

    As the radio tunes across the spectrum, the micro controller is switching IF freq's to keep the received frequency and the used IF frequency as far apart as possible


    as I mentioned way back ;)


    Dave
     
  16. hevans1944

    hevans1944 Hop - AC8NS

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    Opps! I apologize for "stealing" your link. I must have opened it in a new tab while reading your post #9, read the article, then forgot how I "happened" to have it open. <sigh> My short-term memory is becoming worse every day that I age.

    Anyway, I hope the link and the responses from others has helped @Pritam understand IFs a little better.
     
    Last edited: Mar 28, 2016
  17. dorke

    dorke

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    Jun 20, 2015
    Not a problem at all and no need to apologize.
    I just noted that, to point a common view.
    I actually like it when it happens...;)
     
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